Metallic work pieces are joined by a variety of welding processes in which a joint is formed by melting the work pieces and adding a filler material to form a molten pool that cools to form a juncture of increased strength as compared with the strength of the work pieces being joined. One such process that is used in many industrial applications where speed of fabrication is desired is known as gas shielded metal arc welding “GMAW”, also known as inert gas metal welding “MIG”. In this process, robotic welding equipment is used in which the motion of a welding torch is controlled by a programmable controller. The welding torch has a gas shroud containing an electric arc welding tip from which a weld wire electrode projects. An electric arc is produced between the weld wire electrode and the work piece to locally melt the work piece while at the same time; the weld wire electrode is consumed as the filler material. A shielding gas is introduced through the gas shroud to protect the welding area from oxygen and nitrogen in the air that can cause defects in the weld such as porosity and weld embrittlement.
As mentioned above, motion of the welding torch is controlled automatically. This automatic control is dependent upon there being an alignment, namely a straight line, made by the weld wire electrode, the electric arc welding tip and the body of the torch. In more concrete terms, the motion of the torch is controlled relative to a tool center point, also known in the art as a tool control point, through which the weld wire electrode passes. Unfortunately, misalignment within the welding torch can occur due to robot crashes and wear within the electric arc welding tip. Another cause of misalignment is a cast produced within the weld wire electrode itself The weld wire electrode is consumable and therefore, it is supplied to the welding tip by means of a spool of wire. The spool can produce residual stresses that cause the wire to slightly curve as it is dispensed from the electric arc welding tip. All of these sources of misalignment can produce a deviation in the tool center point from weld to weld. As a result, while the programmed motion of the welding torch will not change, the actual position of the weld wire electrode will change to produce weld defects within work pieces to be joined.
In the prior art, in order to automatically detect deviations in the tool center point, a welding equipment station can be provided. Within such station, a contact gage may be provided where the wire touches the gage to ensure properly alignment of the tool center point. Additionally, there are non-contact types of gages in which intersecting light beams are used to test for the presence of the weld wire electrode and thereby verify that the weld wire is at the tool center point. The indication provided by either of these gages allows the operator to suspend operations and perform any necessary maintenance to the welding torch. For example, such maintenance can include replacement of a worn electric arc welding tip so that the weld wire electrode will be at the tool center point when the maintenance is completed. A searching capability can be built into robotic welding equipment using a non-contact type of gage in which after a tool center point test is failed, a search pattern is performed through automated motion of the torch relative to the light beams to measure the actual offset from the tool center point. Such measurement can be used to enable operators to decide whether operations can safely continue with the measured degree of offset or in a manner incorporating a compensation for the offset.
The problem with gages, is that sometimes slight deviations of the tool center point that arise from misalignments of the weld wire electrode that propagate along the length of the weld wire will be missed. As will be discussed, the present invention, among other advantages, provides a robotic welding equipment station that is capable, at low cost, of detecting deviations of the tool center point that arise from such propagating misalignments in a very simple, direct manner.